Cyclodextrin modified microgels as "nanoreactor" for the generation of Au nanoparticles with enhanced catalytic activity

Article abstract of DOI:10.1039/c5ta00197h

We report a facile and green method for the fabrication of hybrid microgels by the immobilization of catalytically active Au nanoparticles in α-cyclodextrin (α-CD) modified poly(N-vinylcaprolactam) (PVCL) microgels without addition of reducing agent and surfactant. It has been shown that only in the case of α-CD modified microgels metal particles were immobilized inside the colloidal gels, which is due to a coordination of the cyclodextrin molecules to the surface of Au nanoparticles. The PVCL-α-CD-Au composite particles can work efficiently as catalyst for the reduction of aromatic nitro-compounds by using the reduction of 4-nitrophenol (Nip) and 2,6-dimethyl-4-nitrophenol (DMNip) as model reactions. Most importantly, due to the selective binding ability of α-CDs to certain reagents, the synthesized hybrid microgels show different catalytic activity for the target compounds during the catalytic reactions: a significant enhancement in the catalytic activity has been observed for the reduction of Nip, while no obvious effect has been found for the reduction of DMNip.

Full text of DOI:10.1039/c5ta00197h

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ARTICLE
Cyclodextrin Modified Microgels as “Nanoreactor”
for the Generation of Au Nanoparticles with
Enhanced Catalytic Activity
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
a
b
a
b
a
He Jia, Dominik Schmitz, Andreas Ott, Andrij Pich, * Yan Lu, *
DOI: 10.1039/x0xx00000x
We report a facile and green method for the fabrication of hybrid microgels by the immobilization of
catalytically active Au nanoparticles in αꢀcyclodextrin (αꢀCD) modified poly(Nꢀvinylcaprolactam)
www.rsc.org/
(
PVCL) microgels without addition of reducing agent and surfactant. It has been shown that only in the
case of αꢀCD modified microgels metal particles were immobilized inside the colloidal gels, which is due
to a coordination of the cyclodextrin molecules to the surface of Au nanoparticles. The PVCLꢀαꢀCDꢀAu
composite particles can work efficiently as catalyst for the reduction of aromatic nitroꢀcompounds by
using the reduction of 4ꢀnitrophenol (Nip) and 2,6ꢀdimethylꢀ4ꢀnitrophenol (DMNip) as model reactions.
Most importantly, due to the selective binding ability of αꢀCDs to certain reagents, the synthesized hybrid
microgels show different catalytic activity for the target compounds during the catalytic reactions: A
significant enhancement in the catalytic activity has been observed for the reduction of Nip, while no
obvious effect has been found for the reduction of DMNip.
Introduction
used polyampholyte poly(Nꢀisopropyl acrylamideꢀcoꢀacrylic
acidꢀcoꢀvinyl imidazole) (poly(NIPAmꢀAAꢀVI)) microgels as
Stimuliꢀresponsive microgels have gained increasing attention
in recent years as they can undergo reversible conformational
changes in response to external stimuli, which are often
accompanied by variations in the physical and chemical
properties of the polymers, such as volume, shape, surface area,
19
carriers to synthesize Au nanorods within the microgels.
Recently Suzuki et al. have synthesized thermosensitive
microgels by using NIPAM and 3ꢀ(methacrylamino)
propyltrimethylammonium chloride (MAPTAC) as the
monomers to immobilize Au nanoparticles. In their study, the
cationic sites in the microgels were used to nucleate particle
1
ꢀ4
solubility, or mechanical properties. More importantly, the
networks of the microgels are wellꢀsuited for inꢀsitu synthesis
of catalytically active metal nanoparticles. Because of the
availability of free spaces in the swollen gel networks, the
stimuliꢀresponsive microgels can provide excellent nucleation
and growth environments for nanoparticles without aggregation.
growth, while NaBH and dimethylamineborane were used as
4
20
the reducing agent. In our previous study, thermosensitive
polystyrene (PS)ꢀPNIPAM coreꢀshell microgels have been
applied to immobilize Ag nanoparticles as well as Au, Rh, Pt
21,22
nanoparticles.
Besides of PNIPAM microgel particles,
5
ꢀ8
Thus, different microgel systems have been developed and
applied as “nanoreactors” for the deposition of metal
PVCL microgels have been also applied as carriers for
inorganic nanoparticles. For example, poly[(
vinylcaprolactam)ꢀcoꢀ (acetoacetoxyethyl methacrylate)]
PVCL/AAEM) microgels have been developed as carriers for
Nꢀ
9
ꢀ12
nanoparticles (NPs).
acrylamide) (PNIPAM) and poly(
are the most commonly used temperature responsive
Among them, poly(
Nꢀisopropyl
N
ꢀvinylcaprolactam) (PVCL)
(
the deposition of ZnS nanoparticles by reaction of zinc acetate
1
3ꢀ16
microgels.
For example, LizꢀMarzán et al. have
23
and thioacetamide under ultrasonic agitation.
successfully combined NIPAM polymerization on the surface
of highly magnetic iron oxide nanocrystals and in situ
nucleation and growth of Ag nanoparticles. They also found
that the temperatureꢀresponsive PNIPAM shell with limited
crossꢀlinking allowed for particularly efficient control of the
In most of these studies, the synthesis of metal nanoparticles
using microgels as template has been always conducted in the
presence of strong reducing agents, such as NaBH , H ,
1
7
4
2
24ꢀ29
NH NH and so forth.
The use of strong reducing agents
2
2
will limit the types of microgels according to the functionalities
within the polymer with respect to chemical stability. Moreover,
1
8
catalysis of the embedded Au nanoparticles. Kumacheva et al.
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Journal of Mater10ials Chemistry A
such approaches provide hybrid structures with a random vinylcaprolactam
distribution of metal nanoparticles inside the microgel structure. (BIS), 2,2’ꢀAzobis(2ꢀmethylpropionamidin) dihydrochloride
Some polymers, such as poly( ꢀvinylꢀ2ꢀpyrrolidone) and (AMPA), trisodium citrate, 2,6ꢀdimethylꢀ4ꢀnitrophenol
(VCL),
N,N’ꢀmethyleneꢀbisꢀacrylamide
N
polyvinylpyrrolidone (PVP), can work both as the reducing (DMNip) and 4ꢀnitrophenol (Nip) were supplied by Aldrich.
agents and the surfactants for the preparation of homogeneous VCL was purified by distillation in vacuo. All other reactants
3
0,31
metal nanoparticles.
work as the “nanoreactors” as observed for microgels. More MilliꢀQ system.
recently, we have reported that a newly developed
However these polymers could not were used without further purification. Water was purified by a
Nꢀ
vinylcaprolactam/acetoacetoxyethyl methacrylate/acrylic acid Synthesis of PVCLꢀαꢀCD microgel
based microgel displays in situ reductive reactivity towards
The synthesis of the aqueous microgel dispersion was done
HAuCl , forming hybrid polymerꢀAu nanostructures at ambient
36
4
using precipitation polymerization according to the literature.
Acrylate modified αꢀcyclodextrin with an average substitution
3
2
temperature without additional reducing agents. However, it is
still a big challenge to develop an active “nanoreactor” based
on microgel particles for the generation of catalytic active metal
nanoparticles containing specific/selective binding domains on
the surface without the use of reducing agents and
surfactant/ligands. This will open new possibilities for the
synthesis of metal hybrid colloidal particles directly under mild
aqueous conditions and facilitate the use for catalytic
applications.
37,38
degree of three was prepared according to the literature
and
used for the synthesis of CDꢀmodified microgels. The
monomers, 2.06 g (14.72 mmol) VCL, 0.05 g AAEM (0.23
mmol), 0.02 g BIS (0.13 mmol) and 0.05 g (0.04mmol) or 0.30
g (0.26mmol) αꢀCD acrylate, respectively, were dissolved in
1
47 mL of water and added to a doubleꢀwall glass reactor with
KPGꢀstirrer. The reaction mixture was purged with nitrogen at
0 °C under stirring (200 rpm). Afterwards 0.02 g (0.07 mmol)
7
In this paper, poly(Nꢀvinylcaprolactam) (PVCL) microgel
AMPA in 3 mL degased water were added. After some
minutes, a turbid dispersion was formed and the reaction was
carried out for 8 h. Obtained microgels were purified by
dialysis using cellulose membranes with a MWCO of 12,000 –
modified with reactive αꢀcyclodextrins (αꢀCDs) has been
synthesized and successfully used to reduce and stabilize Au
nanoparticles in situ without any additional surfactant and
reducing agents. The incorporation of CD molecules into the
microgel leads to a novel “nanoreactor” for metal nanoparticles:
First of all, both PVCL and CDs can work as the reducing
1
4,000 Da. The dialysis was carried out for three days. To
determine the actual αꢀCD content, a freezeꢀdried sample was
analyzed by FTIRꢀspectroscopy.
3
3
agents in the generation of Au nanoparticles. Secondly, the
CD molecules can serve as a stabilizer for the Au nanoparticles
by chemisorption to the nanoparticle surface through the
Synthesis of PVCLꢀαꢀCDꢀAu microgel particles
3
4
The procedure for the synthesis of the PVCLꢀαꢀCDꢀAu
microgel particles is introduced as the follows. In the first step,
hydroxyl groups. The size of Au nanoparticles generated in
the microgels can be controlled. Thus, no additional
surfactant/capping agents are required for the synthesis, which
will efficiently reduce the influence of surfactants on the
surface properties and catalytic activity of Au nanoparticles.
Most importantly, because of the different complexation
abilities of CDs with aromatic nitro compounds with various
an aqueous HAuCl solution (0.2 mL, 0.01 M) was added to 4.8
4
mL PVCLꢀαꢀCD solution (2.12 mg/mL) under vigorous stirring
for 30 min. After that, 0.02 mL NaOH solution (1.0M) was
added to the above mixture for the reduction of HAuCl in the
4
presence of PVCLꢀαꢀCD microgels. The color of the solution
changed slowly from light yellow to wineꢀred. The reaction
lasted for 72 hours under magnetic stirring at room temperature.
After the reaction, the products were cleaned by dialysis.
3
5
structures, the Au nanoparticles could show specific/selective
binding abilities to certain reagents resulting in enhanced
catalytic activity. We will demonstrate this by comparing the
catalytic reduction of 4ꢀnitrophenol (Nip) and 2,6ꢀdimethylꢀ4ꢀ
nitrophenol (DMNip) in the presence of PVCLꢀαꢀCDꢀAu
Synthesis of Au nanoparticles
nanoparticles. The present approach provides a novel and Au nanoparticles with radius around 5nm were prepared
3
9
“
green” route for the fabrication of Au nanoparticles embedded according to the seeds growing method. The seeds solution
in αꢀCD modified microgel “nanoreactors”, which will extend was prepared by adding 0.6 mL of iceꢀcold, freshly prepared
the controlled synthesis of catalytically active metal 0.1 M NaBH4 solution into a 20 mL aqueous solution
ꢀ
4
ꢀ4
nanoparticles with specific surface properties.
containing 2.5*10 M HAuCl and 2.5*10 M trisodium citrate
4
solution and stirring for 4 hours at room temperature. Then 7.5
ꢀ
4
Experimental Section
mL of growth solution (containing 2.5*10 M HAuCl and
4
0
.08M solid cetyltrimethylammonium bromide) was mixed
Materials
with 0.05 mL of freshly prepared 0.1 M ascorbic acid solution.
Afterwards, 2.5 mL of seed solution was added while stirring.
The stirring continued for 10 min after the solution turned into
wine red. The Au nanoparticles were washed by centrifugation
twice before the catalytic reaction.
Gold(III) chloride trihydrate (HAuCl ·3H O), sodium
hydroxide (NaOH), sodium borohydride (NaBH4), solid
cetyltrimethylammonium bromide (CTAB), ascorbic acid
4
2
(
AA),
acetoacetoxyethyl
methacrylate
(AAEM),
Nꢀ
2
| J. Mater. Chem. A, 2015, 00, 1-3
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Catalytic reduction of 4ꢀnitropenol and 2,6ꢀdimethylꢀ4ꢀ
nitrophenol
S1). The synthesized polymer particles showed a decrease in
size for higher contents, while also the swelling ratio decreased
The catalytic activity was investigated as follows. Sodium
borohydride solution (0.5ml, 0.1M) was added to a Nip solution
(
4.5ml, 0.11mM) contained in a glass vessel. The solution was
purged with N to get rid of oxygen from the solution. Then a
2
given amount of PVCLꢀαꢀCDꢀAu particles was added.
Immediately after adding the composite particles, UVꢀvis
spectra of the reaction were taken every 20s in the range of
2
50ꢀ550nm. The catalytic process for DMNip is conducted in
the same way as that of 4ꢀnitropenol as mentioned above.
Characterizations
d
The hydrodynamic radius of the samples as a function of
temperature was conducted by Zetasizer (Malvern Zetasizer
Nano ZS ZEN 3500). FTIRꢀmeasurements were carried out at a
FTꢀIR Nexus (Thermo Nicolet). For quantitative measurements
about 1 mg of sample was weighed, mixed with KBr and
111
)
.
y
PVCL-α−CD(1.03wt.-%)-Au
220
311
i
s
n
t
I
ꢀ
1
PVCL-α−CD(13.08wt.-%)-Au
pressed to a pellet. The CꢀOꢀCꢀabsorption at 1034 cm band
was used for integration and to calculate the CD amount
according to a calibration line. Colloidal stability was analyzed
using a LUMiSizer® from L.U.M. GmbH. The samples were
measured at 4000 rpm over 300 cycles each 30 seconds to
determine the sedimentation velocity. The UVꢀvis spectra were
measured by Lambda 650 spectrometer supplied by Perkinꢀ
Elmer or Agilent 8453. Transmission electron microscope
4
0
60
2Theta (Degree)
80
Figure 1. TEM images of (a) PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu
microgel nanoparticles, (b) PVCLꢀAu microgel nanoparticles
and (c) PVCLꢀαꢀCD(1.03 wt.ꢀ%)ꢀAu microgel nanoparticles.
(
d) XRD patterns of PVCLꢀαꢀCD(1.03 wt.ꢀ%)ꢀAu (black) and
PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu (red).
(
TEM) images were done with JEOL JEMꢀ2100 at 200kV.
XRD measurements were performed in Bruker D8
a
(
see Fig. S2 and S3). As shown in Figure 1a, monodisperse Au
diffractometer in the locked coupled mode (scanning angle
from 10° to 90°) with Cu Kα radiation, the incident wavelength
is 1.5406 Å. For the accomplished measurements the
acceleration voltage is set to 40 kV and the filament current to
nanoparticles with an average diameter of 5~6 nm have been
prepared via the reduction of HAuCl4 by PVCLꢀαꢀCD
microgels without adding any additional reducing agent. The
obtained Au nanoparticles, which are uniform in size and
shape, are homogeneously immobilized in the microgel
carriers. The formation of the Au nanoparticles can be followed
by the change in color from light yellow to wineꢀred of the
solution. As shown in Fig. S4 in the Supporting Information,
the presence of a clear plasmon peak at around 530 nm further
confirms the formation of Au nanoparticles in the presence of
4
0 mA. The amount of Au in the PVCLꢀαꢀCD microgels was
determined by TGA using a Netsch STA 409PC LUXX. Fifteen
o
milligrams of dried sample was heated to 800 C under a
constant argon flow (30 mL/min) with a heating rate of 10
K/min. For the calculation of the surface area (S) of the Au
3
nanoparticles, the density of Au (19.3g/cm ) was used. TGA
and TEM results have been used to obtain the amount and size
of Au particles assuming spherical shape of the Au
nanoparticles. The size distribution of Au nanoparticles was
measured using Image J software based on their TEM images.
At least 100 units were counted.
39
αꢀCD modified PVCL microgels. In a control experiment, no
reduction of Au has taken place in the reaction solution in the
absence of microgel particles. The color of the HAuCl and
4
NaOH mixture was kept light yellow and no obvious
absorbance can be found in the UVꢀvis spectra (Supporting
Information, Figure S4). In addition, PVCL microgels without
αꢀCD modification have been also used to produce Au
nanoparticles. The molecular structure of PVCL is similar with
poly(vinylpyrrolidone), of which the amide group can
transform to enol structure with a hydroxyl group under high
pH conditions. Like longꢀchain alcohols, this kind of polymer
can work as the reducing agent for the synthesis of metal
nanoparticles as well.
Results and Discussion
In situ generation of Au nanoparticles in the PVCLꢀ αꢀCD
microgels
3
6
4
0
PVCL microgels functionalized with αꢀCD units have been
used as an “active” nanoreactor for the in situ generation of Au
nanoparticles by direct reduction of HAuCl4 in an alkaline
aqueous solution. Two samples with different αꢀCD contents
were prepared. The amount of αꢀCD in microgels was
determined to be 1.03 and 13.08 wt.ꢀ%, respectively (see Fig.
4
1,42
As shown in the TEM image in
Figure 1b, Au nanoparticles are generated in the presence of
PVCL microgel particles. However, in this case Au
nanoparticles were hard to be immobilized inside the microgels,
which are weakly attached to the microgel surface or remain as
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secondary particles in solution. On the contrary, in the PVCLꢀ width at half maximum of the (111) Bragg reflection using the
4
3
αꢀCDꢀAu composite particles all of the Au nanoparticles are DebyeꢀScherrer equation. The size thus estimated was 4.7 nm
homogeneously distributed within the αꢀCD modified PVCL for PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu and 4.2 nm for PVCLꢀαꢀ
microgels. We believe that this is due to the reason that αꢀCD CD(1.03 wt.ꢀ%)ꢀAu, respectively, which agrees well with the
can provide an efficient capping on the surface of Au particle size determined from TEM images. Both TEM images
ꢀ
34,51
nanoparticles due to the AuꢀCOO interaction.
In the and XRD patterns demonstrate that the Au nanoparticles
absence of αꢀCD, the interaction between the formed Au reduced by PVCL microgels with different amount of αꢀCDs
nanoparticles and PVCL networks is not strong enough to keep contain almost the same size and crystal structure.
the Au nanoparticles immobilized inside the microgels. Owing Because of the chemisorption between αꢀCD and Au
to the hydroxyl groups in the molecular structure of the nanoparticles (see Fig. 2), the agglomeration of Au
cyclodextrins, the CDs have been reported as the reducing and nanoparticles can be effectively prevented. As shown in Figure
3
3
capping reagent in the synthesis of Au nanoparticles.
3 and Table 1, keeping the amount of αꢀCDs incorporated in the
Interactions of the hydroxyl groups of αꢀCD with the AuꢀCOOꢀ microgels constant, increasing of the amount of HAuCl from
4
surface were monitored by FTIR measurements on a Si crystal. 0.1 ml to 0.4 mL leads to an increase in the Au nanoparticle
KBr pellets were avoided to minimize the water content of the size from 6 nm to 9 nm (the size distribution was shown in
ꢀ
1
sample. A shift of 9 cm to lower wavenumbers of the OHꢀ Figure S7). The UVꢀvis spectra of the PVCLꢀαꢀ CDꢀAu
absorption band for Auꢀloaded microgels is observed nanoparticles prepared at different HAuCl concentrations are
4
(
Supporting Information, Figure S5), which accords with the presented in Figure S8 in the Supporting Information.
34,51
literature reports.
In our study, we have demonstrated for
the first time that the thermoꢀsensitive PVCL microgels
modified with αꢀCD molecules can act efficiently as reducing
agent and capping agent for the synthesis of metal nanoparticles.
Figure 2. Schematic overview of the immobilization of Auꢀ
nanoparticles in αꢀCD modified PVCL microgel.
The effect of αꢀCD amount incorporated in the PVCL
microgels on the formation of Au nanoparticles has been also
studied. Comparing the TEM images shown in Figure 1a and 1c,
it can be seen that no obvious change in shape and morphology
of Au nanoparticles has been found by decreasing the αꢀCD
concentration from 13.08 wt.ꢀ% to 1.03 wt.ꢀ%. Au
nanoparticles with diameter of 5.5±0.3 nm have been generated
in the PVCLꢀαꢀCD(1.03 wt.ꢀ%) microgel particles. Almost all
of the Au nanoparticles are immobilized within the microgels.
Only the amount of Au nanoparticles generated in the microgel
is decreased from 20.07 wt.ꢀ% to 13.26 wt.ꢀ% when decreasing
the concentration of αꢀCD from 13.08 wt.ꢀ% to 1.03 wt.ꢀ%.
Because both of the hybrid microgels were synthesized under
the same conditions except the amount of αꢀCD inside the
PVCL microgels, this nonꢀlinear decrease of Au content against
αꢀCD amount can be explained by the lack of Au precursor
leaving free αꢀCD units that did not bind to Au nanoparticles.
The XRD patterns of these two PVCLꢀαꢀCDꢀAu samples are
Figure 3. TEM images of the PVCLꢀCDꢀAu microgel particles
synthesized with different amount of 0.01 M HAuCl : (a) 0.1
4
ml, (b) 0.2 ml, (c) 0.3 ml and (d) 0.4 ml.
Table 1. Synthesis of PVCLꢀCDꢀAu hybrid particles with
different concentration of HAuCl in the presence of PVCLꢀαꢀ
4
CD (13.08 wt.ꢀ%) microgels (solid content 1.06 wt.ꢀ%).
HAuCl4
NaOH
(1.0M)
PVCL
ꢀαꢀCD
dAu
(nm)
Au loading
(wt.ꢀ%)
(0.01M)
[
a]
(from TEM) (from TGA)
1
2
0.1 ml
0.2 ml
0.01 ml
0.02 ml
1 ml
1 ml
6.0±0.3
16.98
o
o
o
5.9±0.3
20.07
presented in Figure 1d. The peaks at 2θ = 38.34 , 44.22 , 64.88
o
3
4
0.3 ml
0.4 ml
0.03 ml
0.04 ml
1 ml
1 ml
7.7±0.8
9.4±2.2
19.85
21.52
and 77.76 correspond to (111), (200), (220) and (311) Bragg
reflections of Au, which indicates the faceꢀcentered cubic (fcc
)
[
a] A blank TGA experiment was shown in Figure S6.
structure of the synthesized Au nanoparticles. An estimation of
mean size of Au nanoparticles was performed from the full
4
| J. Mater. Chem. A, 2015, 00, 1-3
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It is found that the Surface Plasmon Resonance (SPR) band of colloidal stability in general as they did not sediment
these Au nanoparticles redꢀshifted from 529 nm to 532 nm with completely and showed a relative low sedimentation velocity.
band broadening with the increase of the HAuCl concentration. Only a small decrease in colloidal stability was observed after
4
In addition, there is a continuous increase of the absorption Auꢀloading of the microgel. For a PVCL sample containing
intensity of the absorption band, which is in good agreement 1.03 wt.ꢀ% αꢀCD, the sedimentation velocity increased from
with the finding that the particle size was increased with 0.0466 ꢁm/s for unloaded microgels to 0.0522 ꢁm/s for
increasing HAuCl concentration. When 0.4 mL of HAuCl is particles loaded with Au. For a sample containing 13.08 wt.ꢀ%
4
4
used, some secondary Au nanoparticles have been found in the αꢀCD, the sedimentation velocity increased from 0.0623 ꢁm/s
system as shown in Figure 3d.
to 0.1147 ꢁm/s. This shows that the microgel dispersion does
not lose its stability after deposition of Au nanoparticles, which
is important for its use as catalyst system.
Temperatureꢀresponsive properties and stability of microgel
composites
Catalytic activity of hybrid microgels
As reported in our previous work, PVCLꢀαꢀCD microgels
exhibit temperatureꢀ responsive properties (see in Fig.S2ꢀ3 in
The catalytic reduction of Nip by borohydride has been chosen
as the model reaction to test the catalytic activity of the PVCLꢀ
3
6
the supporting information). With the increasing amount of αꢀ
CDs modified inside the microgels, a more obvious reduction in
the response of the microgels to temperature changes has been
observed. Figure 4 shows the hydrodynamic radii of the
microgel particles before and after Au loading as a function of
temperature.
4
4
αꢀCDꢀAu nanoparticles. The reduction of Nip is one of the
most often used reactions. It can be easily monitored by UVꢀvis
spectroscopy accurately. More importantly, nitrophenol has
been proved to have complexation with cyclodextrin in aqueous
2
9
solution. Such guestꢀhost complex leads to the shift of the
peak of Nip in the UVꢀvis spectra due to the transfer of the
317
321
)
.
u
.
a
(
.
s
b
A
Nip
PVCL-α−CD(13.08wt.-%)-Nip
PVCL-α−CD(13.08wt.-%)-Au-Nip
275
300
325
350
Wavelength (nm)
Figure 4. Hydrodynamic radius of PVCLꢀαꢀCD(13.08 wt.ꢀ%)
microgels with and without Au nanoparticles as a function of
temperature in aqueous solution.
Figure 5. UVꢀvis spectra of Nip mixed with PVCLꢀαꢀ
CD(13.08 wt.ꢀ%)ꢀAu microgels(solid line); PVCLꢀαꢀCD(13.08
wt.ꢀ%) microgels (dot line) and pure Nip (dash dot line).
ꢀ
5
Concentrations: Nip: 10 mol/L; microgels: 0.203 mg/ml.
The Auꢀloaded microgel particles show similar volume phase
transition temperature (VPTT) behavior as that of the PVCL
networks. This result indicates that the immobilization of Au
nanoparticles doesn’t influence the swellingꢀdeswelling
properties of PVCLꢀαꢀCDs microgels and proves additionally
good distribution of Au in the microgels. Moreover, a much
chromophore of the Nip from a polar aqueous environment to a
less polar environment within the αꢀCD cavity.
In our study, UVꢀvis spectra have been taken by mixing Nip
solution with αꢀCD modified PVCL microgel particles. As
shown in Figure 5, the characteristic peak of Nip shifted from
3
5,45
o
sharper volume transition at 27.5 C can be observed for the
3
1
17 nm to 321 nm when PVCL microgel was modified with
3.08 wt.ꢀ% αꢀCD, while no shift can be determined for the Nip
PVCLꢀαꢀCDꢀAu particles. This may be caused by the
chemisorption between Au nanoparticles and αꢀCD molecules
in the microgels, which effectively decreases the possible
amount of αꢀCDs as crosslinking units.
The colloidal stability of the Auꢀloaded sample was compared
with its unloaded pure reference sample at 20 °C. As a measure
for the colloidal stability we determined the sedimentation
velocity of both particle dispersions without and with Au
nanoparticles (Fig. S9 and S10). The samples showed a good
solution mixed with pure PVCL microgels as shown in Figure
S11. Similar shift can be also found for the PVCLꢀαꢀCD(13.08
wt.ꢀ%) microgels after the immobilization of Au nanoparticles.
Moreover, when the amount of αꢀCDs in the PVCL microgels
is too less, the shift of the characteristic peak disappears as
shown in Figure S12 in the Supporting Information for the
PVCLꢀαꢀCD(1.03 wt.ꢀ%) microgels. According to the
3
5
literature, such complexation also exists under alkaline
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Journal of Mater10ials Chemistry A
conditions that the reduction of Nip starts with. In our different amount of PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu hybrid
experiment, when the pH of the solution was set to 10, similar microgel particles at room temperature.
redꢀshift of approximate 3 nm was found in the UVꢀvis spectra
when PVCL was modified with αꢀCD(13.08 wt.ꢀ%) mixing Since the catalysis takes place on the surface of the
with Nip as shown in Figure S13.
The kinetics of Nip reduction in the presence of metal area
nanocomposite particles can be easily followed by UVꢀvis the PVCLꢀαꢀCD microgels. Thus, a kinetic constant k1can be
nanoparticles, the catalytic activity depends on the total surface
S
of the Au nanoparticles immobilized per unit volume of
4
4
spectroscopy. As shown in Figure 6a, after the addition of defined through normalization to the total surface of the
PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu nanocomposites, the peak at 400 nanoparticles in the system:
nm, which is due to the 4ꢀnitrophenate ions, decreases
gradually with time and a new peak appears at 290 nm, which dcNip
=
−
k c
= −k1ScNip
(1)
results from the product 4ꢀaminophenol. Because of an excess
app Nip
dt
ꢀ
46,47
of BH4 the reaction follows a pseudoꢀfirst order reaction.
The ratio of the concentration of the Nip at time to its
original concentration c0 can be directly given by the ratio of
where cNip is the concentration of Nip and
area of the Au nanoparticles in the reaction mixture,
respectively. Here the normalized rate constants k1 have been
used for direct comparison in order to exclude the influence of
S is the total surface
c
t
4
8
4
8
the respective absorbance
A/A0. As shown in Figure 6b, linear
relations between ln( c0) versus time have been obtained in the
c/
ꢀ
1
ꢀ2
particle size. As shown in Figure 7, k1 of 0.025 Ls m has been
determined for the PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu particles,
which is slightly larger than that of PVCLꢀαꢀCD(1.03 wt.ꢀ%)ꢀ
presence of different amount of PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu
catalyst. The apparent rate constant kapp is taken from the slope
of these linear sections.
ꢀ
1
ꢀ2
Au (k1 = 0.020 Ls m ).
PVCL-α−CD(13.08wt.-%)-Au
PVCL-α−CD(1.03wt.-%)-Au
2
1
1
1
.0
.8
.6
.4
1.0
0.4
0.1
0
0
0
0
.015
.010
.005
.000
a
t
0
0
A
/
A
k
app
)
.
1.2
1
-
0
250
500
u
.
s
t (s)
a
(
1.0
0.8
0.6
0.4
0.2
0.0
.
s
p
p
a
b
A
k
0.0
0.2
0.4
0.6
0.8
250
300
350
400
450
500
2
S
(
m /L
)
Wavelength (nm)
Figure 7. Rate constant (kapp) as a function of surface area S of
Au nanoparticles normalized to the unit volume of the system:
PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu (solid squares) and PVCLꢀαꢀ
CD(1.03 wt.ꢀ%)ꢀAu (solid circles) at room temperature.
1
0
0
.0
.4
.1
b
Since the size and crystal structure of the Au nanoparticles in
these two microgels are identical, the difference of the reaction
constants for these two samples may be due to the reason that
the amount of the αꢀCDs grafted in the PVCL microgels is
different. As shown schematically in Figure 8, the
complexation of Nip with αꢀCDs that are attached on the Au
particle surface favors the adsorption of Nip to the surface of
Au nanoparticles. This will increase the local concentration of
Nip on the Au nanoparticles surface, which leads to the
increase of the catalytic rate constant. This effect will be more
obvious when more αꢀCDs are incorporated into the PVCL
microgels. So the PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu particles show
higher catalytic activity compared to PVCLꢀαꢀCD(1.03 wt.ꢀ%)ꢀ
Au particles. Meanwhile, according to the literature, the
complexation between αꢀCD and the reduction product 4ꢀ
0
C
/
C
-1
2
1
.22*10 m /L
-1
2
1
2
3
.75*10 m /L
-1 2
.27*10 m /L
-1
2
.32*10 m /L
0
50
100
150
200
250
300
350
t (s)
Figure 6. (a) UVꢀvis absorption spectra of Nip reduced by
sodium borohydride using PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu
particles as catalyst at room temperature. The inset shows
typical time dependence of the absorption of 4ꢀnitrophenolate
ions at 400 nm. (b) Kinetic analysis of the Nip reduced by
6
| J. Mater. Chem. A, 2015, 00, 1-3
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Page 7 of 10
Journal of Materials Chemistry A
Journal of Materials Chemistry A
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ARTICLE
DOI: 10.1039/C5TA00197H
49
aminophenol (Amp) is not so stable as the Nip. As shown in The reduction of DMNip by NaBH has been monitored by
4
Figure S14, no shift of the characteristic peak can be observed UVꢀvis spectroscopy that the characteristic absorption peak of
for the 4ꢀaminophenol solution mixed with the PVCLꢀαꢀCD DMNip at 431 nm weakens with reaction time and the
(
13.08 wt.ꢀ%) microgels, which means that it is hard for the αꢀ absorption peak of 2,6ꢀdimethylꢀ4ꢀaminophenol (DMAmp) at
CDs to form hostꢀguest complex with 4ꢀaminophenol. This 293 nm increases gradually, which indicates the reduction of
indicates that the product cannot influence the reaction speed.
DMNip to DMAmp as shown in Fig.S16 in the Supporting
5
2
Information. As shown in Figure 9, a linear relation between
kapp and the surface of the Au nanocatalysts can be observed
S
for the reduction of Nip and DMNip. The reaction constants k1
of different catalysts for both catalytic reactions have been
summarized in Table 2 and can be compared with each other
directly. First of all, the Au nanoparticles immobilized in the
PVCLꢀαꢀCD microgels show much higher catalytic activity
than that of other catalyst systems for both catalytic reactions.
This demonstrates that PVCLꢀαꢀCD microgels can work
efficiently as reducing and capping agents for the generation of
Au nanoparticles with high catalytic activity. More
interestingly, it is clearly to see that the rate constant for the
ꢀ
1
ꢀ2
reduction of DMNip (k1 = 0.018 Ls m ) is slower than that of
ꢀ
1
ꢀ2
Nip (k1 = 0.025 Ls m ) when PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu is
used as catalyst. This is opposite to the result of the CTABꢀ
stabilized Au nanoparticles, which do not have complexation
with Nip as shown in Figure S18 in the supporting information.
Figure 8. Immobilization of Nip on the αꢀCDꢀmodified Au
nanoparticle surface.
ꢀ
1
ꢀ2
In order to further prove that the CD modified microgels
contain certain catalytic selectivity for the target compounds,
the catalytic reduction of 2,6ꢀdimethylꢀ4ꢀnitrophenol (DMNip)
by sodium borohydride in the presence of PVCLꢀαꢀCD(13.08
wt.ꢀ%)ꢀAu microgels has been studied. DMNip has the similar
structure with Nip but greater steric hindrance, which weakens
the ability of αꢀCDs to form hostꢀguest complex with DMNip
In that case, the catalytic reduction of Nip (k1 = 0.008 Ls m )
ꢀ
1
ꢀ2
is slower than that of DMNip (k1= 0.014 Ls m ). Similar result
also has been reported by K. S. Suslick that the reaction rate of
Nip is slower than that of DMNip when using Au nanoparticles
5
2
encapsulated in porous carbon as catalyst.
Table 2. Rate constant k1 of different catalysts for the reduction
of 4ꢀnitrophenol (Nip) and 2,6ꢀdimethylꢀ4ꢀnitrophenol
5
0
molecules.
(
DMNip).
Microgels-Nip
Microgels-DMNip
Au-Nip
Reduction of
0
0
0
0
0
.008
.006
.004
.002
.000
Reduction of Nip
dAu (nm)
(from TEM)
Sample
ꢀ1 ꢀ2
DMNip
k
1
(Ls m )
ꢀ1 ꢀ2
k
1
(Ls m )
Au-DMNip
PVCLꢀαꢀ
CD(13.08 wt.ꢀ
)ꢀAu
0.025
0.018
5.9±0.3
%
1
-
PVCLꢀαꢀ
CD(1.03 wt.ꢀ%)ꢀ
Au
s
0.020
ꢀ
5.5±0.3
p
p
a
k
CTABꢀstabilized
Au
[a]
0
.008
0.014
10.9±0.4
[
b]
ꢀ4
ꢀ4
USP Au/C
2.76*10
3.31*10
31
[
a]
TEM images of Au nanoparticles have been shown in Figure
0
.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
S17
2
S
(
m /L
)
[
b]
USP: Ultrasonic spray pyrolysis; calculated according to the
data from the reference
Figure 9. Rate constant kapp of the PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀ
Au and pure Au nanoparticles for the catalytic reduction of 4ꢀ
nitrophenol (Nip) and 2,6ꢀdimethylꢀ4ꢀnitrophenol(DMNip) by
52
The enhancement in the catalytic reduction of Nip in the
presence of PVCLꢀαꢀCD(13.08 wt.ꢀ%)ꢀAu nanoparticles is due
to the complexation of Nip with αꢀCDs, which increases the
local concentration of Nip on the Au nanoparticle surface. Inthe
case of DMNip, this complex interaction is much weaker.
That’s why the reaction rate is slower compared to that of Nip.
This indicates that dependent on the complexability of the
compounds with the αꢀCDs, the incorporation of αꢀCDs into the
sodium borohydride as a function of surface area
nanoparticles normalized to the unit volume of the system.
S of Au
This has been confirmed from the UVꢀvis spectra that no
obvious shift for the characteristic peak of DMNip has been
observed when it is mixed with PVCLꢀαꢀCD(13.08 wt.ꢀ%)
microgels as shown in Fig.S15 in the Supporting Information.
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microgels will provide Au nanoparticles with specific binding
abilities to certain compounds leading to enhanced catalytic
activity.
1
M. A. C. Stuart, W. T. S. Huck, J.Genzer, M. Müller, C. Ober, M.
Stamm, G. B. Sukhorukov, I. Szleifer, V. V.Tsukruk, M. Urban, F.
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2
3
4
5
6
7
Conclusions
4
,
,
Au nanoparticles have been synthesized inꢀsitu within the αꢀCD
modified PVCL microgels without adding any reducing agents
and surfactants. The size and loading degree of the Au
3
3
nanoparticles can be controlled by the HAuCl amount. The
4
Z. Wang, B. Tan, I. Hussain, N. Schaeffer, M. F. Wyatt, M.Brust, A.
I. Cooper, Langmuir, 2007 , 23 , 885.
presence of αꢀCD in the microgels allows homogeneous
distribution of Au nanoparticles in colloidal polymer networks.
In addition, the immobilization of Au nanoparticles doesn’t
influence the swellingꢀdeswelling properties of the PVCLꢀαꢀ
CD microgels. Similar VPTT behavior of the PVCL networks
has been observed for the PVCLꢀαꢀCDꢀAu hybrid particles.
After deposition of Auꢀnanoparticles inside the microgels, the
hybrid particles still preserve good colloidal stability. The
PVCLꢀαꢀCDꢀAu composite particles can work efficiently as
catalyst for the reduction of aromatic nitroꢀcompounds. Most
importantly, due to the ability of αꢀCDs to form complexes with
specific compounds, the synthesized hybrid microgels show
different catalytic activity for the target compounds during the
catalytic reactions. A significant enhancement in the catalytic
activity has been observed for the reduction of Nip, while no
obvious effect has been found for the reduction of DMNip.
Considering the selective binding/complexation properties of
CDs with a variety of different guest molecules together with
the reducing and stabilizing properties of the PVCLꢀαꢀCD
microgels, the novel hybrid microgels developed in the present
study could create new opportunities for functional
nanomaterials with possible applications in catalysis.
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Acknowledgements
,
He Jia gratefully acknowledges financial support of CSC
scholarship. DS and AP thank VolkswagenStiftung and
Deutsche Forschungsgemeinschaft (DFG SFB 985 “Functional
Microgels and Microgel Systems) for financial support.
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For Table of Contents Use Only
Cyclodextrin Modified Microgels as “Nanoreactor” for the
Generation of Au Nanoparticles with Enhanced Catalytic Activity
a
b
a
b
a
He Jia, Dominik Schmitz, Andreas Ott, Andrij Pich,* Yan Lu*
a
Soft Matter and Functionmal Materials, HelmholtzꢀZentrum Berlin für Materialien und Energie, HahnꢀMeitnerꢀPlatz 1, 14109 Berlin, Germany.
Functional and Interactive Polymers, Institute of Textile and Macromolecular Chemistry, RWTH Aachen University, DWI Leibniz Institute for
b
Interactive Materials, Forckenbeckstr. 50, Dꢀ52056 Aachen, Germany.
αꢀCyclodextrin modified poly(Nꢀvinylcaprolactam) microgels have been applied as “nanoreactors” for the generation of AuNPs
with enhanced catalytic activity for the reduction of 4ꢀnitrophenol.
This journal is © The Royal Society of Chemistry 2013
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